1. Field of the Invention
The present invention relates to an optical system for measuring the human cornea. The system includes a light source, a scanner, an optical deflecting system, a sensor and an evaluating unit. The system is used by placing the eye in front of the optical deflection system.
2. Description of the Related Art
A measuring system of the above-described type is known particularly from DE-T1-35 90 539. This document was first published in the English language as WO/86/02249. The reference discloses an arrangement of a laser measuring system including scanner, sensor and evaluating unit. A holographic element is used as a focusing device closely in front of the eye. This holographic element is selected in such a way that, in the case of an ideal positioning of an ideal cornea, the beam reflected by the cornea surface coincides with the incoming beam. Accordingly, for evaluating the data, it is necessary to place in the path of the beam a beam divider which deflects a portion of the reflected beam onto an area sensor. This beam divider simultaneously deflects a portion of the beam directed by the laser through a scanner onto a second two-dimensional sensor whose output signals inform a computer as to how the emitted light beam is directed at a given moment. From the data and from the knowledge of the stationary geometry or the geometry at a given moment, the computer is capable of computing the shape of the cornea surface.
However, this known arrangement has several significant disadvantages.
The holographic element must be of a large size in order to always effect the theoretically correct angle of incidence on the eye. Only when this requirement is met, is it possible to obtain the measurement result in the range of the area sensor and to obtain a measured value in this manner.
The area sensors themselves limit the measurement accuracy and dissolution and are expensive. In order to obtain the desired measurement accuracy, it is necessary to select appropriately large spacings. However, it is not only necessary to consider the beam expansion on the positively curved cornea, but it must also be taken into consideration that, in the case of injury to the surface of the cornea, significant deviations of the surface normal from the ideal condition can occur, so that it is possible that several measurement points successively do not provide a measurement value because the reflected beam can no longer reach the area sensor.
The beam divider represents another significant disadvantage. While the beam divider advantageously offers the possibility of providing the computer with information concerning the position of the beam at a given moment, the beam divider simultaneously weakens the beam reflected by the eye, so that it is necessary to radiate the eye with an intensity which is sufficient for ensuring that the reflected beam after the partial reflection at the beam divider is still reliably capable of appropriately activating the area sensor. However, this increased light intensity is an extremely unpleasant additional burden to the eye.
Additional problems are due to the fact that the length of the path of the beam from the scanner to the reference area sensor is significantly shorter than the length of the path of the reflected beam. The angle of incidence and, thus, the angle of the emitted beam from the beam divider is the same as the corresponding angle of the reflected beam, however, the emitted beams and, thus, also the image thereof on the reference area sensor, diverge over the entire measurement range, as seen projected over the beam divider. This does improve the measurement accuracy, however, again represents a limitation in the possibilities of arranging the scanner, the beam divider and the reference area sensor.
DE-OS 25 24 152 discloses another optical system, although intended for measuring large plane surfaces. In this system, a rotating light beam is reflected by stationary deflecting mirrors in such a way that the light beam covers during its rotation the entire area to be scanned. However, the reference does not disclose the manner of the actual determination of the measurement values, i.e., nothing is disclosed concerning reflected beam, type of scanner, etc.